Problem/Motivation
We would like to have expandable menu item where we like
Steps to reproduce
1-Add a sub-menu under the default "My Account" item of the "User account menu" menu (make sure to use an item from the "User account menu" not the "Main Menu"
2-Make sure that the parent of the newly created menu item is set as "Show as expanded" and make sure that the block containing this menu have the proper options to display it as expanded (option "Number of levels to display").
Result:The two menu items are stacked one on the other... The arrow to expand it is missing.
Proposed resolution
Here is a first hint:
If I replace the code of templates/navigation/menu.html.twig (which is displaying the staked menu items) by the code of templates/navigation/menu--primary-menu.html.twig Then the behavior of the menu becomes what was expected.
Conclusion 1: it looks like the problem is coming from templates/navigation/menu.html.twig
Conclusion 2: Maybe there is an additional problem because the padding and the position of the expanded menu of the "Main Menu" is not the same as the one of the "User account menu"
See the padding/position of the menu item in the following capture and compare with the upper screenshot: